1. Field of the Invention
The invention relates to a heat exchanger for vehicles. The heat exchanger has a heat exchanger core provided with a row of distanced heat exchanger tubes open at their ends and at least one dosed container, arranged in the tube end regions, for the distribution of a medium flowing through the heat exchanger tubes. Each container includes a tube holding element and a container closure element, which are connected to each other in a sealed manner over connection sections, and whereby the heat exchanger tubes are put through the tube holding element, connected to it and form spaces between each other.
2. Related Technology
For a new generation of heat exchangers with an environmentally friendly, yet very efficient refrigerant, particularly based on carbon dioxide at high pressure (R744), the burst pressure requirements of the heat exchanger assemblies, particularly of the two containers, for reasons of safety will rise to more than three times the current amount during the burst test, with a simultaneous rise of the temperature from today's 20° C. to 180° C. (high pressure side) or 120° C. (low pressure side). In order to counteract the burst danger during the present day production of a heat exchanger, particularly of a condenser, the wall thickness of the container closure element and the tube holding element ought to be increased by at least 2 mm.
The heat exchanger tubes contain pressurized flowing media, which can deliver or take up heat, and thus depending on their function are cooling tubes or heating tubes.
The current problems are:
In ratio of the active depth (baffle plate and multiple passage tubes) to the depth of the tube holding element and container closure element, for example, for a 12 mm baffle plate/tube package, approximately double the depth is quoted for the tube holding and container closure elements connected to each other. Due to twice the depth, too much space is required in the vehicle for the heat exchanger.
Because the widely varying material weights in the assembled container it will be difficult to find the suitable brazing conditions which ensure that the entire material will be provided with the proper brazing, temperatures without melting the thinner baffle plates. The range of the brazing parameters is therefore more than critical.
The heavy wall thickness between the different row chambers of the heat exchanger, for example, of an evaporator, results in too wide gaps between the multiple passage tube rows. That leads in this region to long thermal transfer paths from the refrigerant to the passing air. In this region the baffle plate efficiency is relatively low.
Thus the high pressure requirements essentially imply material problems.
As shown in a sectional view in FIG. 6a traditional heat exchangers have containers that consist of two elements—a tube holding element and a container closure element. The tube holding element is usually a brazing-plate flat material, which has holes stamped to receive the heat exchanger tubes and is bent in a mold. The container closure element is often an extruded profile. Baffle plates can be provided between the heat exchanger tubes to increase the transfer surface in the heat exchanger core.
A problem of the containers is their structure, whereby heavy wall thickness of the tube holding element and the container closure are necessary, in order to lower the danger of bursting of the containers, to withstand the given medium pressure, particularly a higher burst pressure, or pressure peaks, respectively. Additionally, the wall thickness require relatively big dimensions of the container and hence of the heat exchanger, which therefore demands much space in the vehicle.
A heat exchanger is known from U.S. Pat. No. 3,993,126, where the containers are provided with several single strengthening ribs between the tube holding element and the container closure element. On the other hand, to reinforce the tube holding element a distribution plate made of plastic material is hidingly provided in the region of the tube holding element, whereby the distribution plate is provided with a plurality of insertion holes which are arranged correspondingly with the insertion holes of the tube holding element. High demands of the plastic material exist to establish the container, in order to withstand a high pressure in the container.
A structure of the tube holding element is described in U.S. Pat. No. 4,381,033, where the inner wall of the tube holding element is provided with at least one U-shaped holding element attached to a portion of the rear side to support a baffle wall, which is insertable into the holding element and is in connection with the container closure element. The baffle wall divides the container into an entry chamber and an exit chamber for the coolant. The heat exchanger is provided for the use of traditional coolants. The baffle wall can hardly withstand a higher coolant pressure.
Another heat exchanger is described in U.S. Pat. No. 5,236,044, the containers of which are constructed of the above mentioned elements, where there are engagement holes for dividing baffles to produce container chambers for the deviation of the coolant into the associated heat exchanger tubes and for counterflow passage. The connection section between the tube holding element and the container closure element is the true weak spot, when the container internal pressure is increased. To make the tube holding element withstand the high pressure of the medium flowing in the containers, it is to be provided with extensions of the end regions on the long sides, whereby the extensions are to cover the previous connection sections and be in connection to the container closure elements such that the extensions bear against the rear side of the container closure elements in an overlapping and holding way. A problem with the above is that the overlapping reinforcement is not sufficient to withstand the high pressure of modern medium substances, particularly of a carbon dioxide (R744) flow under pressure.
Another heat exchanger is described in U.S. Pat. No. 5,605,191, the container of which consists of a face plate and a cover. The container volume is divided into two chambers by at least one separating element. The associated face plate has a plurality of put-through holes, through which the heat exchanger tubes are put. Further, the face plate has fit grooves made between the insertion holes for accepting the end sections of the heat exchanger tubes. The heat exchanger tube sections in the end region project into the interior of the container. The separating element is provided with recesses and insertions and, prior to container assembly and brazing, is put on the end sections of the heat exchanger tubes. The insertions are fit in the fit grooves. The rear side of the separating element is to bear against the cover, which is then attached to the tube holding element end regions. A problem is that with a minimum distanced arrangement of the separating element at the inner wall of the cover the container cannot guarantee safety against the high burst pressure requirements, particularly when a highly pressurized medium such as R744 is used.
Another heat exchanger is disclosed in U.S. Pat. No. 5,806,587, where the structure of a container with strengthening plates is described as single metal pieces, separated from each other or as ribs at wide distance, are put in between the tube holding element and the container closure element to reinforce the container. A problem with the above is that the container cannot withstand pressure in case of real high pressure applications with carbon dioxide. In addition, problems may arise during brazing of the plates at the container elements.
A container or collector of a heat exchanger for motor vehicles provided with a chamber division created by crossing flat webs is described in U.S. Pat. No. 6,082,448. The collector consists of a tube bottom, where the heat exchanger tubes are guided, and a dosing cover. The flat webs for the division into chambers have separate holding plug arrangements on both sides to arrange them in the insertion holes of the cover and the tube bottom. The chambers serve to guide and turn back the flowing medium in neighboring heat exchanger tubes. Problems arise due to several complicated processes to braze the elements to each other and when leakages occur in the chambers due to higher pressure of the flowing medium. The chambers are not given with their dimensions related to the increased pressure.
A fluid cooling device is described in U.S. Pat. No. 6,223,812 as a heat exchanger with two containers and heat exchanger tubes arranged as layers between the containers, where each container includes a container wall and a tube insertion wall with a plurality of openings into which the heat exchanger tubes are inserted. At the connection point between the container wall and the tube insertion wall parallel to the tube layers there is a connection batten, from which finger-like projections extend to the outside of the container. The fingers engage between the free distance regions of the layers heat exchanger tubes. The fingers can be connected to both the outside of the tube insertion wall and the outer walls of the heat exchanger tubes and together with the connection batten form an outer comb towards the heat exchanger core strengthening the tube insertion wall. While the normal stability between the tube insertion wall and the inserted heat exchanger tubes is improved, a problem is that an increased pressure in the container can hardly be withstood. The sensitive spot loaded by the inner pressure is the transition region from the container closure element to the connection batten. An operating pressure of only twofold maximum the traditional medium operating pressure can be reached in the chamber in the container.
Another problem of the heat exchangers manufactured to the state-of-the-art is that the integration of the heat exchanger tubes into the tube holding elements is by putting in the heat exchanger tubes into the tube holding elements across the cylinder axis of the tube holding elements or the cylinders established by the collectors/distributors. The heat exchanger tubes therefore deeply project into the cylinder due to the curvature in radial peripheral direction. This raises fluid dynamic problems and strength problems of the heat exchanger in the whole.